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| CERT C++ Secure Coding Standard | STR37-CPP. Arguments to character handling functions must be representable as an unsigned char |
| ISO/IEC TR TS 17961 (Draft) | Passing arguments to character-handling functions that are not representable as unsigned char [chrsgnext] |
| MITRE CWE | CWE-704, Incorrect type conversion or cast CWE-686, Function call with incorrect argument type |
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| [ISO/IEC 9899:2011] | Section 7.4, "Character Handling <ctype.h>" |
| [Kettlewell 2002] | Section 1.1, "<ctype.h> and Characters Types" |